Steam is supplied to a turbine at high pressure and temperature from a boiler and the energy in the steam is converted into mechanical work by expansion through the turbine. The expansion of the steam takes place through a series of static blades or nozzles and moving blades. An annular row of static blades or nozzles and its associated annular row of moving blades is referred to as a turbine stage. After the steam has been expanded in a high pressure (HP) turbine, it is conventional to return it to the boiler for re-heating and then to return the steam to an intermediate pressure (IP) turbine, from which the steam exhausts through one or more low pressure (LP) turbines.
An impulse turbine stage is one in which all or most of the stage pressure drop takes place in the row of static blades. The steam jet produced does work on the rotor of the turbine by impinging on the following row of moving blades, which are aerodynamically designed to provide a low reaction. In practice, impulse stages are designed with a small pressure drop over the moving blades (e.g. 5–20% degree of reaction, which is the percentage of the stage enthalpy drop taken over the moving blades).
A reaction turbine stage is one in which a substantial part (e.g. roughly half or more) of the stage pressure drop takes place over the row of moving blades. For example, reaction blading may be designed with a 50% degree of reaction, which gives approximately equal pressure ratios over the static and moving rows.
In a turbine with impulse blading, it is conventional to use a disc-type rotor, the static blade assemblies constituting diaphragms that extend into chambers between the rotor discs. The diaphragms extend radially inwards to a small diameter, for efficient sealing against the rotor due to the smaller leakage flow area.
In a turbine with reaction blading, the pressure drop over the static blade assembly is considerably less than over the static blade assembly of an impulse stage, and it is conventional to use a drum-type rotor. An outer static ring of the static blade assembly is radially keyed to the turbine casing so as to move with the casing. The moving blades have root portions carried within slots in the periphery of the drum.
FIG. 1 of the accompanying drawings shows a known type of disc and diaphragm arrangement. A turbine rotor 1 comprises a series of discs 2 with annular chambers 3 between them. Each disc 2 carries an annular row of moving blades 4, each having a root 6 fixed to the disc 2 by pins 7. The static blade assembly or diaphragm 8 which is immediately upstream of the disc 2 (with respect to the steam flow direction indicated by the arrows 9) comprises an annular row of static blades 11 extending between a radially outer static ring 12 and a radially inner static ring 13. The outer ring 12 is housed in and axially located by the turbine casing 14 and has an axial extension 16 carrying a fin-type labyrinth seal co-operating with the shrouds 18 of the moving blades 4. In this instance, the labyrinth seal comprises an axial series of circumferentially extending strips 17 whose hooked ends are caulked into an axial extension 16 of the outer static ring. The inner ring 13 (which is more massive than the outer ring 12) is accommodated in the chamber 3 between two discs 2 and carries a fin-type labyrinth seal 19 restricting the leakage flow (indicated by arrows 21) past the diaphragm 8. In this instance, the labyrinth seal 19 comprises an axial series of circumferentially extending, alternately longer and shorter triangular- or knife-section fins that extend from the seal carrier towards sealing lands on the rotor surface. The seal carrier itself is segmented to allow the seal 19 to have a limited degree of self-adjustment in the radial direction.
FIG. 2 of the accompanying drawings shows a known type of turbine with a drum-type rotor 22, the diameter of the periphery 23 being substantially constant. Each annular row of moving blades has the root portions 26 of the blades fixed in circumferentially extending slots in the rotor 22. As in FIG. 1, the shrouds 27 of the moving blades 24 are again sealed to the turbine casing 28 by fin-type labyrinth seals. In each annular row of static blades 29, an outer shroud portion of each blade is individually mounted in a circumferential slot in the casing 28 as shown. Their inner shroud portions are provided with inner fin-type labyrinth seals 32 adjacent the periphery 23 of the rotor 22. A disadvantage of this arrangement is that the outer shroud portions move with the casing 28 as it expands and contracts.